A high frequency circuit for mobile phones is provided with a filter or a duplexer. The filter or the duplexer often includes acoustic wave resonators such as a surface acoustic wave resonator, a love wave resonator, a boundary acoustic wave resonator, and a film bulk acoustic resonator.
FIG. 1A is a plan view schematically illustrating an exemplary structure of the surface acoustic wave resonator. FIG. 1B is a cross-sectional view taken along a line Z-Z in FIG. 1A. In the surface acoustic wave resonator illustrated in FIGS. 1A and 1B, a pair of comb-shaped electrodes 102 are formed on a surface of a piezoelectric substrate 101. On both sides of the comb-shaped electrodes 102, grating reflectors 103a and 103b are provided.
FIG. 2A is a plan view schematically illustrating an exemplary structure of the love wave resonator. FIG. 2B is a cross-sectional view taken along a line Z-Z in FIG. 2A. The love wave resonator illustrated in FIGS. 2A and 2B is obtained by depositing a first dielectric 104 on the surface acoustic wave resonator including the piezoelectric substrate 101, the comb-shaped electrodes 102, and the grating reflectors 103a and 103b. Patent Document 1 (JP 2004-112748 A) discloses the love wave resonator as illustrated in FIGS. 2A and 2B.
FIG. 3A is a plan view schematically illustrating an exemplary structure of the boundary acoustic wave resonator. FIG. 3B is a cross-sectional view taken along a line Z-Z in FIG. 3A. The boundary acoustic wave resonator illustrated in FIGS. 3A and 3B is obtained by depositing the first dielectric 104 and a second dielectric 105 on the surface acoustic wave resonator including the piezoelectric substrate 101, the comb-shaped electrodes 102, and the grating reflectors 103a and 103b. Patent Document 2 (JP 10(1998)-549008 A) discloses the boundary acoustic wave resonator as illustrated in FIGS. 3A and 3B.
FIG. 4A is a plan view schematically illustrating an exemplary structure of the film bulk acoustic resonator. FIG. 4B is a cross-sectional view taken along a line Z-Z in FIG. 4A. In the film bulk acoustic resonator illustrated in FIGS. 4A and 4B, an upper electrode 202, a lower electrode 203, and a piezoelectric film 204 are formed on a substrate 201. The piezoelectric film 204 is sandwiched between the upper electrode 202 and the lower electrode 203. An excitation portion 206 is a region where the upper electrode 202 and the lower electrode 203 face each other. A through hole 205, an air gap, and the like are provided below the excitation portion 206.
FIG. 5 illustrates exemplary admittance characteristics of the acoustic wave resonators illustrated in FIGS. 1 to 4 in the vicinity of a main resonant frequency. The acoustic wave resonators illustrated in FIGS. 1 to 4 have double resonance characteristics with a main resonant frequency (fr0) and a main antiresonant frequency (fa0). The main resonant frequency (fr0) and the main antiresonant frequency (fa0) have values close to each other.
FIG. 6A is a circuit diagram of a ladder-type filter in which any of the acoustic wave resonators illustrated in FIGS. 1 to 4 are connected in a ladder shape. Connecting acoustic wave resonators 300 in a ladder shape as illustrated in FIG. 6 achieves a ladder-type filter having bandpass characteristics in which high frequency components and low frequency components are suppressed as illustrated in FIG. 6B.
FIG. 7 illustrates the principle on which the bandpass characteristics of the ladder-type filter are obtained. In FIG. 7, a solid line indicates the bandpass characteristics obtained when the acoustic wave resonator alone is connected in series (hereinafter, referred to as a series resonator). The series resonator forms a low pass filter having a turnover frequency between a resonant frequency (frs) and an antiresonant frequency (fas). In FIG. 7, a dashed line indicates the bandpass characteristics obtained when the acoustic wave resonator alone is connected in parallel (hereinafter, referred to as a parallel resonator). The parallel resonator forms a high pass filter having a turnover frequency between a resonant frequency (frp) and an antiresonant frequency (fap). The resonant frequency (frs) of the series resonator and the antiresonant frequency (fap) of the parallel resonator are set to be substantially equal to each other. The ladder-type filter, which includes the series resonators and the parallel resonators, has the bandpass characteristics as illustrated in FIG. 6B as a result that the characteristics indicated by the solid line and the characteristics indicated by the dashed line in FIG. 7 are synthesized.
Since the resonance phenomena of the acoustic wave resonator occur due to mechanical vibrations, they often include not only main resonance (main antiresonance) but also several kinds of sub-resonance (sub-antiresonance) corresponding to various vibration modes.
FIG. 8 illustrates exemplary admittance characteristics of the acoustic wave resonator. FIG. 8 illustrates frequency characteristics in a wide frequency band including sub-resonant frequencies. As illustrated in FIG. 8, in a frequency band away from the main resonant frequency (fr0) and the main antiresonant frequency (fa0), a sub-resonant frequency (fr1) and a sub-antiresonant frequency (fa1) exist. Further, in a frequency band away from the sub-resonant frequency (fr1) and the sub-antiresonant frequency (fa1), a sub-resonant frequency (fr2) and a sub-antiresonant frequency (fa2) exist. The number of occurrences of the sub-resonance (sub-antiresonance) varies depending on the type of the acoustic wave resonator. Further, a frequency interval between the sub-resonance and the main resonance varies depending on the type of the acoustic wave resonator. Even in the acoustic wave resonators of the same type, however, the number of occurrences of the sub-resonance (sub-antiresonance) and the frequency interval between the sub-resonance and the main resonance may vary depending on the material, film thickness, and the like of the acoustic wave resonator.
As described above, since the acoustic wave resonator produces the sub-resonance, the filter using the acoustic wave resonators accordingly has passbands formed based on the sub-resonance.
FIG. 9 illustrates exemplary bandpass characteristics of the ladder-type filter using the acoustic wave resonators in a wide frequency band. As illustrated in FIG. 9, the ladder-type filter using the acoustic wave resonators has not only a main passband (referred to as a main resonant response) formed due to the main resonance, but also passbands (referred to as sub-resonant responses) formed due to the sub-resonance. While the acoustic wave resonator generally is specified to suppress frequency components in frequency bands other than the main passband formed due to the main resonance (hereinafter, referred to as suppression specifications), the suppression specifications may not be met due to the sub-resonant responses as illustrated in FIG. 9.